1. Field of the Invention
The present invention relates to a copper foil most suitable for a fine pattern circuit board, especially for chip-on-film (COF) use or for a high-frequency printed circuit board, and a copper foil most suitable for an electromagnetic shield of a plasma display panel (PDP). Further, the present invention relates to a printed circuit board, a PDP electromagnetic shield, and a high-frequency printed circuit board using this copper foil.
2. Description of the Related Art
Along with the reduction in size and weight of electronic devices, recent electronic parts have been made highly integrated. Corresponding to this, the circuit patterns of printed circuit boards also have had to be made higher in density. Circuit patterns of a fine line width and pitch are now being formed. In particular, IC boards for driving the liquid crystal displays of display units of personal computers, mobile phones, and personal digital assistants (PDA) are being made increasingly higher in density. As ICs are being directly mounted on substrate films, these are called as “chip-on-film” (COF) structures.
In COF mounting, the IC position is detected by light passing through the film formed with the copper foil interconnect patterns. However, the recognition rate of a conventional electrodeposited copper foil for a printed circuit board (ability to detect IC position by light) is very poor. The reason is that the surface of the copper foil is rough. That is, the parts of the film through which the light penetrates are parts from which unnecessary copper foil other than the copper circuit parts are etched away. Unevenness of the surface of the copper foil is transferred and remains on the surface of the film when adhering the copper foil on the film. The unevenness of the film surface therefore becomes larger. Due to the unevenness, when the light passes, the amount of the light able to proceed straight becomes smaller. Therefore, the recognition rate becomes poor.
Conventional copper foil for printed circuit boards is poor in this recognition rate. Therefore, a two-layer material comprised of a polyimide film on which copper layer is formed by sputtering (thin film deposition process) and which is further copper plated is used. However, this sputtered copper layer has only a weak bonding strength to film. Therefore, there was the possibility of the phenomenon that the etchant or plating solution penetrates into the boundary between the copper foil and the film, i.e., the “undercut phenomenon” during the circuit manufacturing process. Further, due to the weak bonding strength between sputtered copper layer and the film, there is the danger of the copper foil peeling off from the substrate during use as a product.
Ordinarily, electrodeposited copper foil produced by an electrodeposited foil production apparatus as shown in FIG. 1 is treated to prevent rust and to roughen its surface to increase bondability by a surface treatment apparatus as shown in FIG. 2. The electrodeposited foil production apparatus comprises a rotating drum-shaped cathode 2 (surface made of SUS or titanium) and an anode 1 (lead or precious metal oxide coated titanium electrode) arranged concentrically with respect to the cathode 2. A current is passed across the electrodes while circulating an electrolyte 3 to cause the copper to deposit on the surface of the cathode 2 to a predetermined thickness, then the copper is peeled off from the surface of the cathode 2 as copper foil 4. In the present specification, the copper foil at this stage is called the “untreated copper foil 4”, the surface of the untreated copper foil 4 contacting the electrolyte 3 is called the “matt surface”, and the surface contacting the rotating drum shaped cathode 2 is called the “gloss surface”.
The untreated copper foil 4 is improved in performance required for a printed circuit board, that is, the bondability to a resin substrate, by being passed through the surface treatment apparatus shown in FIG. 2 and subjected the continuous steps of electrochemical or chemical surface treatment, that is, roughening, metal plating, stainproofing, and silane coupling treatment. In these surface treatments, the roughening is ordinarily applied to the matt surface of the untreated copper foil. The resultant copper foil is used as a surface treated copper foil 8 for a printed circuit board. Note that, in FIG. 2, reference numerals 5 and 6 indicate the electrolyte and reference numeral 7 indicates an anode of the surface treatment apparatus.
One of major factors influencing the etching rate and recognition rate of the surface treated copper foil 8 is the roughness of the copper foil surface. For the etching rate, it is important that the roughness of the matt surface and the gloss surface of the copper foil both be small. For the recognition rate, it is important that the surface roughness of the side to be bonded to the film be small. Further, copper foil to be used for a PDP electromagnetic shield is also required to be a copper foil having a small surface roughness. A PDP is a display panel using emission of light by electrodischarge and is comprised of a glass vessel evacuated and then filled with mercury gas etc. Voltage is applied to cause electrodischarge. The ultraviolet light generated at this time strikes fluorescent coating previously applied to the inside of the glass vessel to cause emission of visible light. PDP has mainly been used for display applications at train stations and public facilities or exhibitions and family restaurants, but recently demand for household use large-scale televisions has been increasing.
In a PDP, leakage of the electromagnetic waves generated from the device is prevented by the arrangement of an electromagnetic shield at the front of the display (side close to viewer). As an electromagnetic shield for electromagnetic waves leaking from devices used in public facilities, use has been made of a sheet of glass provided on its entire surface with a thin film of silver, a fine nylon mesh material plated by copper, etc. However, in a home television, further reduction of the electromagnetic waves generated is required. This has become difficult with a conventional shield. Therefore, a type of shield comprised of a transparent film such as a polyester film on which copper foil is bonded and etched to leave a mesh of copper on the film has come into use. This shield is arranged in front of the display. The viewer views the light passing through parts without the mesh copper. To obtain a good image quality, the mesh has to have a good light transmission. Shields having a copper foil mesh of a width of 10 μm and a pitch of 200 μm are now being mass produced.
Since this mesh is produced by etching, the requirements on the copper foil for a PDP are similar to the requirements on copper foil for a high density printed circuit board. That is, in order to produce a copper foil mesh having a fine width, it is necessary that the roughness of the matt surface and the gloss surface of the copper foil both be small. A copper foil having a small surface roughness is required. Further, electronic devices are not only being reduce in size and weight, but also are being designed for further higher frequencies along with the increase of information transfer. Up to now, a high frequency signal over 1 GHz had been used for restricted radio use such as communications for aircraft and satellite communications, but recently is being used also for familiar electronic devices such as mobile phones and wireless LAN. The printed circuit boards for such high frequencies have to use a resin superior in high frequency characteristic. The copper foil has to be one having less high-frequency transfer loss. That is, the higher the frequency, the more noticeable the “skin effect” where a signal passes primarily at the surface part of a conductive layer. Due to this, when using a conventional copper foil having a large unevenness, the high-frequency transfer loss is large. Therefore, it can no longer be used for such applications.
A major factor influencing the high-frequency transfer loss is the surface roughness. The higher the frequency, the more noticeable the skin effect, so the roughness of the matt surface and the gloss surface of the copper foil both have to be small for the high-frequency transfer loss. There are two major factors influencing the roughness of the matt surface of the copper foil. One is the surface roughness of the matt surface of the untreated copper foil. Another is the method of deposition of granular copper in the roughening. If the surface roughness of the matt surface of the untreated copper foil is large, the surface roughness of the copper foil after roughening becomes large. The amount of deposition of the granular copper in the roughening can be adjusted by the current in the roughening. But the surface roughness of the untreated copper foil is primarily decided by the electrodeposition conditions when the copper to be deposited on the above drum-shaped cathode 2 in FIG. 1 and by the additives added to the electrolyte 3.
General electrodeposited copper foil is roughened on its matt surface to a surface roughness of about 6 μm at a thickness of 12 μm and about 10 μm at a thicker copper foil of 70 μm in terms of the surface roughness Rz (Rz as defined in JIS B0601-1994, “Definition and Description of Surface Roughness”, 5.1 “Definition of 10-Point Average Roughness”). When the copper foil bonding surface for bonding with the resin substrate has a relatively large surface roughness in this way, a high bonding strength where copper particles which had been present on the roughened surface of the copper foil and part of the copper foil deposited in a branch-like manner deeply penetrate the resin substrate is obtained. On the other hand, it takes time to dissolve the copper particles and deposited copper in a branch-like manner penetrating the resin substrate completely in etching to form printed circuits resulting in the occurrence of so-called “etching residue”. As a result, the bottom lines between the copper foil and the resin substrate become poor in linearity and insulation between adjacent circuits becomes poor when the distance between circuits is made narrower. In notable cases, a circuit is unable to be fabricated completely and the phenomenon of bridging of adjacent circuits occurs.
Further, the gloss surface (surface of side contacting drum) has gloss at first glance and seems to be smooth, but becomes a replica of the drum surface. The roughness is usually about 1.5 to 2.0 μm in terms of Rz in average. This is because manufacture is started in a state with the initial drum surface polished smooth, but later the drum surface is dissolved and gradually becomes rough along with continuance of manufacture of the electrodeposited copper foil due to the electrolyte being strongly acidic. If after manufacture of the electrodeposited copper foil for a certain time the drum surface becomes rough, the drum surface is polished again to smooth it, but seen on an average, the roughness is about 1.5 to 2.0 μm.
If the surface roughness is large, the bondability of the dry film etching resist bonded to the copper foil surface becomes locally good and locally bad. Therefore, the circuit is sometimes formed in a wave shape during etching. If becoming wavy in this way, the circuit becomes poor in linearity and the problem arises that it becomes difficult to form fine patterns. Further, in a case of using a liquid resist, while the degree is slight compared with a dry film resist, due to the difference of the dissolution rate between depressions and projections of the copper foil, the phenomenon of the shape of the circuit becoming wavy is seen in the same way. To deal with the requirement of increasing the pattern fineness, in addition to the roughness of the matt surface of the copper foil being small, it is important that the roughness of the gloss surface also be small.
Due to the above reasons, it is important that the surface of the copper foil be small in surface roughness of the side to be bonded with the film for the recognition rate and it is important that the surface roughness be small at both sides for a copper foil for use in a plasma display panel and high-frequency printed circuit board. Above, the importance of surface roughness was described with reference to an electrodeposited copper foil, but a rolled copper foil also has the same problems of course.